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> VHFHomepage >Charge Control| How does a Ni-Cd battery work? |  |
| What happens when cells are charged? | |
| What’s so tricky? | |
| It looks like batteries are well protected. Now what? | |
| I need to properly charge the cells. How do I do that? | (See Below) |
Q: How does a Ni-Cd battery work?
A:
Let us answer this with another question. How does an electrochemical cell work?
First, a bit of nomenclature. A cell is a single electrochemical device with a
single anode and a single cathode. A battery is a collection of cells, usually
connected in series to obtain a higher terminal voltage. Batteries, whether they
are primary (use once) or secondary (rechargeable) are devices, which convert
chemical energy into electrical energy. In the case of the latter, they can take
electrical energy and store it as chemical energy for later use. The key to
electrochemistry is the process of oxidation and reduction. Remember the
phrase” “LEO (the lion) goes GER (grr??)”—Lose Electrons
Oxidation—Gain Electrons Reduction. When one oxidizes a material, it gives up
electrons it becomes more positively charged, or enters a higher oxidation
state. Likewise, when one reduces a material, one is adding electrons to it and
either making it negatively charged or reducing its oxidation state. Now, one
can make a cell using two materials, say A and B and immersing them in a
solution, which can conduct ions, called an electrolyte. (An ion is a charged
atom or radical of a molecule capable of transferring electrical charge). Now,
let us say that material A is easily oxidized—it likes to lose electrons,
while B is a material that likes to be reduced. When these two materials are
immersed in an electrolyte, and a circuit is completed from A to B, A is
oxidized and electrons are released to flow to the circuit. After performing
electrical work, the electrons flow into B, where B is reduced. The circuit from
B to A is completed by the flow of ions in the electrolyte. A secondary cell can
be reversed by forcing electrons into A, and reducing the oxidized A to regain
unoxidized A for use again. This, of course, is an oversimplified view, as only
certain combinations of materials and electrolytes provide useful and practical
batteries. Oh, one more bit of nomenclature: The cathode is where reduction
takes place, and the anode is where oxidation takes place. So, in a battery,
which is producing current, the positive terminal is the cathode, and the
negative terminal is the anode. Yes, this is counterintuitive from our
understanding of diodes, where the cathode is negative with respect to the
anode... Now, the NiCd system itself: When the cell is fully charged: The
cathode is composed of Nickelic Hydroxide. Now, nickel is one of those elements
that has multiple oxidation states—it can lose a different number of electrons
per atom, depending on how hard it is coerced. Nickel is usually found with
oxidation states of 0 (free metal), +2, +3 and +4. The +2 state is referred with
a -ous suffix, while the +3 and +4 states are referred with a -ic suffix. So,
nickelic hydroxide is really NiOOH (the nickel has a charge of +3) or Ni(OH)
(the nickel has a charge of +4) 4 The anode is composed of free cadmium metal
(zero oxidation). The electrolyte is usually a solution of potassium hydroxide
(KOH). When one connects a load to the cell, as explained earlier, the anode is
oxidized and the cathode is reduced. Electrons leave the anode where the cadmium
is oxidized and forms Cd (OH), plus 2 free electrons. 2 These two electrons go
to the cathode where they reduce the nickelIC hydroxide to form nickelOUS
hydroxide or Ni (OH) (where the nickel has 2 a charge of +2) This reaction can
take place until the materials are exhausted. In theory, cells are manufactured
so that both anode and cathode are spent at roughly equal rates.
Q:
What happens when cells are charged?
A:
Well, in a nutshell, the inverse of the discharge. To charge, one is forcing
current back into the cell (opposite of discharge current). Here, electrons are
being taken out of the positive terminal, and forced into the negative terminal.
This means that the material at the positive terminal is being oxidized (hence
is now the anode—confusing, eh?) and material at the negative terminal is
being reduced (now the cathode). In the NiCd system, the cadmium hydroxide is
being reconverted into cadmium, and the nickelous hydroxide is being reconverted
to nickelic hydroxide. Note that the electrolyte in both charge and discharge is
a means to move the hydroxyl (OH-) ions around. Unlike the lead-acid system, the
electrolyte really doesn’t change in composition too much between the charged
and discharged state.
Q:
What’s so tricky?
A:
The easy part of charging is reconverting the spent material on the plates to
the charged condition. The hard part is knowing when to stop. Let us take a
moment to think about what happens when we overcharge the battery. Once all the
nickelous hydroxide is converted into nickelic hydroxide, and in theory all the
cadmium hydroxide is converted into cadmium, the charging current has to go
somewhere. As the energy of the charging current cannot go into more chemical
energy, it goes into splitting water (water is still the major constituent of
the electrolyte). Just like the age old chemistry experiment of splitting water
into hydrogen and oxygen, a fully charged NiCd cell does the same thing. You are
forcing oxidation at the positive terminal and reduction at the negative. When
one oxidizes water (actually the OH-) ion, one produces oxygen. Likewise, at
the negative terminal (now the cathode), one produces hydrogen. This of course
is bad. Oxygen + hydrogen = BOOM. Cell manufacturers, or at least their
lawyers, frown on this from happening. So, they cheat. During manufacture, they
deliberately oversize the negative plate, and they partially discharge it. That
is, they put a fully charged positive plate, but put a slightly discharged, but
bigger plate of cadmium in. The amount of free cadmium in the oversized plate is
matched to discharge in step with the amount of nickelic hydroxide provided in
the positive plate. Now consider what happens as full charge is achieved.
Oxidation of water starts at the anode, but since the cathode is oversized, and
has excess hydroxide, the current continues to produce cadmium metal instead of
hydrogen. At the same time, the separator (the material used to prevent the
plates from shorting) is designed to allow oxygen gas to diffuse through, from
the positive to the negative plate. The free oxygen then oxidizes the cadmium
metal to form more cadmium hydroxide to prevent hydrogen from being formed.
Voila—a safe battery.
Q: It looks like batteries are well protected. Now what?
A: Not so fast..... this scheme will work only as long as the overcharging current is limited to a value such that the rate of oxygen liberation at the anode is less than or equal to the rate of diffusion across the separator. If the overcharging current is too high, excess oxygen is produced at the anode, and since not enough oxygen can diffuse across to make up for the reduction at the cathode, the excess cadmium hydroxide is used up. Then, hydrogen is formed. This leads to a dangerous situation, due to both fire and overpressure. Cells are designed to vent when this condition occurs, releasing the excess hydrogen and oxygen to the air before really bad things happen. While this may keep one’s cells from blowing up, it does damage them, since one is losing material from one’s cell. As one loses water, it upsets the chemical balance inside the cell—lose enough water, and it stops working. Another problem is that the process of generating oxygen, and recombining it at the cathode generates heat. With a moderate amount of current, the cell temperature can rise considerably, to 50 or 60 degrees C. If after charging, the batteries are hot, then you have overcharged them—slap yourself on your wrist.
Q: I need to “properly” charge cells. How do I do that?
A: There are many methods of charging. One is trickle or the old 15-hour method. This involves using a current of about 50 mA (for AA cells) and leaving them on charge for 15 hours. At this current level, oxygen diffusion is more than enough to take care of the excess current once full charge is achieved. Of course, one runs the risk of voltage depression due to overcharge. The best method is the so-called delta-V method. If one plots the terminal voltage of the cell during a charge with a constant voltage, it will continue to rise slowly as charging progresses. At the point of full charge, the cell voltage will drop in a fairly short time. The amount of drop is small, about 10 mV/cell, but is distinctive. There are circuits out there built specifically to look for this. The Maxim MAX712 and 713 ICs are ones that come to mind now. This method is expensive and tedious, but gives good reproducible results. There is a danger in this though. In a battery with a bad cell this delta - V method may not work, and one may end up destroying all the cells, so one needs to be careful. If one ends up putting in more than double the charge capacity of the cell, then something is wrong. Another cheap way is to measure the cell temperature. The cell temperature will rise steeply as full charge is reached. When the cell temperature rises to 10 degrees C or so above ambient, stop charging, or go into trickle mode. Whatever method one chooses, a failsafe timer is a requirement with high charge currents. Don’t let more than double the cell capacity of charge current flow, just in case. (i.e. for a 800 mAh cell, no more than 1600 mAh of charge).
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